The present invention relates to a semiconductor photodetector for detecting light of a predetermined wavelength, and in particular, for detecting light relatively rapidly of the predetermined wavelength. The invention also relates to a switch incorporating the semiconductor photodetector of the invention.
In picosecond optical chronoscopy, correlation based measurements are often made for determining the presence of pulsed light. Such correlation measurements require a material with high efficiency for a non-linear detection of light, for example, lithiumniobate for second harmonic detection, where a first pulse of light, namely, the light the presence of which is to be detected, to be compared with a second pulse of light or with a delayed version of the first pulse, in order to measure the temporal characteristics of that first pulse of light. If the second light pulse with which the first light pulse is being compared is a delayed version of the first light pulse, then the process is referred to as correlation. If, on the other hand, the second light pulse with which the first light pulse is being compared is derived from a light source different to that from which the first light pulse emanated, then this process is properly referred to as cross-correlation. However, in general, a distinction between correlation and cross-correlation is often not made, and both forms of correlation are referred to as correlation. In this specification the use of the term “correlation” is intended to refer to both auto and cross-correlation. Correlation measurements can be performed in two ways, most common is a time averaged method where the average signal from a regular pulse train is measured using a slow non-linear detector, since due to the low efficiency of the non-linear processes used, real time correlation measurements can be made where the correlation signal is measured in real time, but requires a high efficiency of the non-linear process.
Devices for carrying out correlation measurements of light signals which comprise laser pulses of picoseconds and sub-picoseconds duration are known. For example, recently semiconductor devices which operate on the principle of Two-Photon Absorption have been demonstrated to be suitable for carrying out time averaged correlation measurements. Such an arrangement is described by Skovgaard et al, in 1998 Optics Communications, Vol. 153, pp. 78-82, ‘Two-photon conductivity in semiconductor waveguide autocorrelators’. In general, such Two-Photon Absorption semiconductor devices do not provide high sensitivity characterisation of light signals of short optical pulses in real time, since such devices require that the light of the light signal be of relatively high optical intensity in order to obtain a significant level of photocurrent and to exceed residual linear absorption, and thus to provide the sensitivity required. One known technique for increasing the sensitivity of Two-Photon Absorption devices where the light intensity of the light signal is relatively low, is to provide the light with a relatively long interaction length with the two photon absorbing material. However, this results in a reduction in the operating speed, and thus the detecting speed of the device, thus rendering such devices unsuitable for either real time or time averaged correlation measurement in the pico/sub-pico seconds range. Since correlation measurement devices are attractive for use in high speed telecommunications, which require operation in the picosecond/sub-picosecond range, known Two-Photon Absorption devices with relatively long active lengths are unsuitable for such requirements.
There is therefore a need for a semiconductor photodetector for detecting light of a predetermined wavelength which is suitable for both correlation and cross-correlation, and indeed, for other uses for detecting light of predetermined wavelength, which overcomes this problem, and there is also a need for such a device which is suitable for use in relatively high speed systems, for example, high speed telecommunication systems.